Isolated USB cable, hub

2010/03/01

The next design I’m showing off is a tiny little board (1.4″ x 0.45″) that houses both an ADuM4160 USB 2.0 low/full-speed isolator, and an ADuM5000 100mW power isolator. The two together form the core of a fully-isolated USB cable, which means you can safely connect your externally-powered device to your computer without any worries about where the relative ground potentials are. This is critical for my main contract, because each board takes power from further and further down a main power bus, which means that even after the regulators do their thing, the ground potential on each subsequent node on the wire is different from the previous.

The ADuM4160 requires power on both sides of the transformer isolation barrier to run its encode/decode circuitry, which is why the ADuM5000 is required to provide power. The upstream port of a USB device does not provide any power to the cable, so there’s no power for the device side of the isolator without the ADuM5000. However, the 100mW limit means you really can’t run most bus-powered devices off the isolator. OTOH, why would you be putting a ground isolator on a bus-powered device anyway…?

The other design is a 7-port USB hub based on the same concept. The upper-left port is the upstream, the remainder are downstream. All 8 ports are fully isolated for both USB and power. The 100mW limit of the upstream ADuM5000 should be sufficient to drive the 7 downstream ADuM4160/ADuM5000 pairs as well as the TUSB2077A hub chip, but not provide any power at all to even the lowest-power of devices…

I haven’t had luck yet getting the cable isolator to work, due to a combination of problems not the least of which was the lack of proper silkscreen in the parts library footprint I stole. It helps if the ADuM5000 isn’t backwards, eh? I haven’t gotten back to trying to finish that because the memories of spending a week bashing my head bloody against the problem are still too painful. I’ll get to it soon though, because I will need a quantity of them very shortly. The hub is still sitting on my desk as a bare PCB, since I don’t really want to start sticking some $70 of parts on the board until I know the smaller version works…

Once they are confirmed working, I plan to offer them for sale somehow. I know there’s interest, because only a few days after posting to the DorkbotPDX list about them, I got an email from someone entirely unrelated to dorkbotpdx about wanting to buy some. Go Google!

[…] will sag inward from 0-V+ over distance). I’ve made several attempts to solve this problem (isolated USB, wireless debugger) and so far they’ve all fallen short for various reasons (none of which […]